Silicon carbide (SiC) is a class of compound semiconductors which are thermally and chemically stable. Compared to silicon (Si), SiC has advantageous physical properties such as a band gap which is approximately three times as large, a dielectric breakdown voltage which is approximately ten times as large, an electron saturation velocity which is approximately twice as large, and a coefficient of thermal conductivity which is approximately three times as large. On account of these excellent properties, SiC is expected to be used as a material for electronic devices which can overcome the limitations due to physical properties of Si devices, such as power devices and hostile-environment devices which operate at high temperatures.
In the field of optical devices, nitride-type materials (GaN and AlN) are being developed in order to make it possible to decrease the wavelength applied. Compared to other compound semiconductor materials, SiC has much smaller lattice mismatching with nitride-type materials, so it has attracted attention as a material for substrates on which a nitride-type material is grown.
Accordingly, there is a demand for good quality SiC single crystal wafers which can be used as devices or substrates and for an efficient manufacturing method therefor. To meet this demand, it is necessary to manufacture good quality SiC bulk single crystals (single crystal ingots) for use in the manufacture of wafers.
[Manufacture of SiC Bulk Single Crystals]
SiC single crystal wafers are manufactured by slicing wafers from a SiC bulk single crystal and processing the wafers by lapping (grinding to a uniform thickness), polishing (mirror finishing), and the like. These processes can be carried out in the same manner as for the manufacture of Si single crystal wafers, and they are known to those skilled in the art.
Known methods of manufacturing SiC bulk single crystals used in the manufacture of SiC single crystal wafers include the sublimation recrystallization method and the solution growth method.
SiC single crystal wafers which are currently commercially available are mainly manufactured by the sublimation recrystallization method. In this method, SiC powder which is a raw material is sublimated at a high temperature of 2200-2500° C. in a crucible made of graphite or the like, and a SiC single crystal is recrystallized on a seed crystal made of a SiC single crystal disposed in a low temperature region inside the crucible.
A SiC single crystal which is grown by the sublimation recrystallization method has the problem that it contains dislocations and micropipe defects which are continued from dislocations and micropipe defects in the seed crystal as well as a large number of dislocations which are thought to be generated during crystal growth. The following are thought to be causes of the generation of new dislocations during crystal growth. (1) The sublimation recrystallization method is basically a reaction which progresses inside a closed system in a crucible, so the composition of the sublimation gas which is supplied by sublimation of the SiC raw material varies during crystal growth. (2) Since the reaction takes place in a solid phase and a vapor phase, a large temperature gradient is present in the growth environment, and as a result, large thermal stresses develop in the crystal. (3) As crystal growth progresses, the growth interface moves within the crucible, so the temperature environment and the concentration of the sublimation gas which is a raw material vary over time.
As growth progresses, new defects are generated due to the above-described variation in crystal growth conditions. Therefore, it is extremely difficult to obtain a single crystal having a quality greatly exceeding that of the seed crystal by the sublimation recrystallization method. Furthermore, in the sublimation recrystallization method, because it is difficult to obtain a high purity raw material SiC powder or a high purity crucible, the SiC single crystals which are recrystallized are unavoidably contaminated by impurity elements such as boron and nitrogen. These impurities may induce dislocations within a SiC crystal, and it becomes difficult to produce a low-doped layer, which is necessary for the manufacture of devices, with good controllability.
In the solution growth method, carbon (C) is dissolved in a melt of Si or a Si alloy to prepare a SiC solution in the melt which serves as a solvent. The dissolution of C is allowed to proceed until the SiC dissolved in the solution is in a state of thermodynamic equilibrium with solid phase SiC (namely, the concentration of SiC in the solution reaches a saturated concentration). A SiC seed crystal is brought into contact with the resulting SiC solution (liquid phase), and a supersaturated state of SiC is formed at least in the vicinity of the seed crystal by supercooling the solution, thereby causing a SiC single crystal to grow on the seed crystal. A typical method of forming a supersaturated state is the so-called temperature difference method in which a temperature gradient is formed so that the temperature of the melt in the vicinity of the seed crystal is lower than the temperature in other areas of the melt.
The solution growth method, which is a liquid phase growth method, has excellent temperature controllability because the growth temperature can be lowered by around 500-1000° C. compared to the sublimation recrystallization method. Therefore, thermal stresses inside the crystal being grown can be made extremely low, and the occurrence of dislocations can be suppressed. Furthermore, crystal growth takes place in a state close to thermodynamic equilibrium, and it is possible to substantially eliminate variations in factors such as the composition of the solution during crystal growth. As a result, the generation of new dislocations during crystal growth can be nearly entirely eliminated, and it is possible to manufacture a good quality SiC bulk single crystal having markedly fewer dislocations or micropipe defects compared to when using the sublimation recrystallization method.
[Manufacture of SiC Single Crystal Epitaxial Wafers]
In order to manufacture SiC devices using a SiC single crystal wafer (also referred to as a bulk wafer) which is manufactured from a SiC bulk single crystal (single crystal ingot) manufactured by the above-described sublimation recrystallization method or solution growth method, the SiC single crystal wafer is used as a substrate, and on its surface, it is necessary to form a low-doped SiC single crystal epitaxial film or a single crystal epitaxial film of a Group III-V compound semiconductor including a nitride semiconductor such as GaN, with the epitaxial film being precisely controlled with respect to its concentration of impurities and thickness. A wafer having such an epitaxial film is referred to as an epitaxial wafer.
Known methods of forming a SiC epitaxial film for manufacturing an epitaxial wafer include the chemical vapor deposition (CVD) method and the liquid phase epitaxy (LPE) method. In the CVD method, silane gas and a hydrocarbon gas, which are raw material gases for SiC, are made to undergo thermal decomposition on a substrate to deposit a SiC film on the substrate. The LPE method is nearly the same as the above-described solution growth method. Namely, a SiC solution formed by dissolving C in a melt of Si metal or a Si alloy with at least one other metal is used as a liquid phase, and a substrate is brought into contact with the solution. The SiC concentration of the solution at least in the vicinity of the substrate is made a supersaturated state, thereby causing a SiC epitaxial film to grow on the substrate. In the LPE method, since crystal growth takes place in a state close to thermodynamic equilibrium, the density of crystal defects can be reduced.
It is known that the surface of a SiC single crystal wafer used as a substrate for epitaxial growth has an affected surface layer which is formed by transformation of the crystal structure of a good SiC single crystal. The affected surface layer is a layer containing a native oxide film or a subsurface damaged layer. A native oxide film is an oxide film which is formed in the surface of a SiC single crystal in the atmosphere. A subsurface damaged layer is a layer having damages in crystal structure which are introduced during the working process for manufacturing a SiC single crystal wafer from a bulk single crystal.
If an epitaxial film is grown on a SiC single crystal wafer having such an affected surface layer, regardless of which of the above-described methods for growing an epitaxial film is used, the quality and properties of the resulting epitaxial film are degraded. Therefore, a surface layer of a SiC single crystal wafer is removed prior to epitaxial growth. A known method for removing the surface layer is performed by removal of the damaged surface layer by oxidation followed by removal of the resulting oxide film. However, it is difficult to remove the oxide film without causing damages.
Patent Document 1 (JP 06-188163 A1) discloses a SiC single crystal substrate in which an affected surface layer of a SiC single crystal wafer is removed by means of dry etching. It is described therein that damaged portions which are dispersed in the surface of a wafer are uniformly removed by reactive ion etching (RIE) with an etching depth of 200-400 nm. It is also reported that if the etching depth exceeds 400 nm, the wafer surface becomes rough.
Patent Document 2 (JP 09-183700 A1) discloses a method of removing deep affected surface layers scattered on a wafer without producing surface roughening of the substrate while maintaining the flatness existing prior to etching. In this method, the wafer surface is etched using an ionized inert gas, and the resulting subsurface damaged layer which is newly formed by ion irradiation is removed by etching with a reactive gas. However, this method requires a plurality of etching steps, so it is inefficient.
Non-Patent Document 1 [Jpn. J. Appl. Phys., 40, 3315 (2001)] discloses carrying out in situ etching (namely, etching inside an epitaxial growth apparatus) of a SiC single crystal using H2 gas to remove a surface layer of a wafer prior to epitaxial growth by the CVD method. When an epitaxial film is formed by CVD, this method is convenient since H2 etching can be carried out inside a CVD apparatus, and it is currently widely used to remove an affected surface layer from SiC wafers. However, H2 etching is difficult to apply to a method other than the CVD method (such as the LPE method) or to an apparatus for such other method. In addition, it is known that the density of surface defects which are generated when carrying out epitaxial growth greatly varies in accordance with the H2 etching conditions and that surface defects are unavoidably generated in an epitaxial film even if etching conditions are optimized.
In actuality, if an affected surface layer of a SiC single crystal wafer like that described above is removed by a conventional removal method and then an epitaxial film is formed, there are still factors which degrade the quality of the epitaxial film. These factors cause a decrease in throughput (productivity) when manufacturing semiconductor devices from SiC single crystal wafers. From the standpoint of increasing throughput, there is a strong demand to identify the main factors causing a decrease in the quality of an epitaxial film and to eliminate them.